The present invention relates to a device for measuring scattered light from a measurement volume with compensation for background signals.
The measurement of scattered light is a well-known and trusted method for determining the concentration of dust or other particles in gaseous media. For instance, it is used in the measurement of the emissions of combustion plants. For that matter, a light beam coming from a sensor head is emitted into a measurement volume, which is scattered back by the particles. This scattered light is detected by the sensor head and is a measure for the particle concentration in the measurement volume.
In the measurement of scattered light, the measured power of the scattered light is very small in relation to the irradiated power. By multiple reflections and scattering, the light emitted by the sensor head generates background light in the surroundings of the measurement volume, which is also detected by the sensor head. For this reason, the sensor head provides a measurement signal resulting from the scattered light from the measurement volume, and a background signal resulting from background light. The strength of the background signal is in the order of magnitude of the measurement signal. In measurements within a chimney or another enclosed space, a strong background signal is caused by reflections and scattering on the wall opposite to the sensor head (called “background wall” in the following).
It is known to suppress the background light by a light trap, that is to say by a suitable device on the background wall opposite to the sensor head. It is furthermore known to measure the background light by a separate control receiver and to compensate it by calculation.
The documents DE 27 54 139 A1 and CH 600 456 describe a smoke detector, which uses both of these procedures. In one embodiment, two radiation receiving units are arranged behind on single lens. The fields of view of the two radiation receiving units have their focus at two neighbouring points, one of which is located within a radiation beam of a light transmitter. The two fields of view partly overlap each other on the opposing wall. The light transmitter is arranged vertically to the optical axis of the lens. Due to the only partial overlap of the fields of view, any complete compensation of the background radiation, which the light receiver directed to the measurement volume receives, is not possible.
The document EP 1 983 328 B1 describes a measurement device for determining a particle concentration within a measurement range, which works also with a measurement unit having a measurement receiver for the measurement signal and a control unit with a control receiver. When the device is being built in, the control receiver must be aligned such that it looks onto the same background as the measurement receiver. For the adjustment of the measurement device when it is being started or for maintenance works, a changing device is built in into the measurement unit and into the control unit, by which the measurement receiver and the control receiver can be changed by an adjustment light source, so that the effective cross section of the beam of the control light lobe and the measurement light lobe is visible for the human eye. The adjustment is sumptuous. In the adjustment, the beam cross sections are made coincident on a background wall of that room in which the measurement volume is situated. New adjustment is necessary in every new measurement situation. Moreover, the high space requirement of the optics is cumbersome when a measurement has to be performed across an opening in a wall. Finally, the adjustment light sources must be aligned exactly to the measurement receiver and the control receiver, which necessitates a high installation effort.